Silicon carbide (hereinafter referred to as "SiC") has a wide band gap, and its maximum breakdown electric field is larger than that of silicon by one order of magnitude. Thus, SiC has been highly expected to be used as a material for power semiconductor devices in the next generation. Up to the present, single crystals, such as 6H--SiC and 4H--SiC, have been developed and manufactured with considerably high quality, and various types of semiconductor devices, including Schottky diodes, vertical MOSFET, and thyristors, have been fabricated, using SiC as a semiconductor material. It has been confirmed that these devices exhibit far better characteristics than conventional Si semiconductor devices.
SiC is known as growing an SiO.sub.2 film on its surface to provide a desirable interface between a semiconductor substrate and an insulating film, by a thermal oxidation method similar to that employed for silicon. Since the SiO.sub.2 film thus formed can be used as a gate insulating film or stabilizing film, SiC may be equally applied to MOS type semiconductor devices. The above property, i.e., formation of SiO.sub.2 film, is peculiar to SiC, and cannot be observed in other compound semiconductor materials. Thus, MOS type semiconductor devices, such as MOSFET, can be easily manufactured utilizing this property, and SiC is expected to be used in a wide range of applications in the future.
The present invention is concerned with an ion implantation step that is essential to the manufacture of the above-indicated semiconductor devices. The ion implantation technique, which makes it easy to form regions having different impurity concentrations and conductivity types in a semiconductor crystal, is essential to the fabrication of semiconductor devices. The above-described SiC devices are also fabricated using the ion implantation technique.
In the case of SiC, for example, nitrogen or phosphorous is generally used for forming n type regions, and aluminum or boron is generally used for forming p type regions. After the ion implantation, the SiC substrate needs to be annealed or heat-treated at a high temperature for activation of the impurities thus implanted in the substrate. While the annealing temperature for silicon ranges from 900.degree. C. to about 1250.degree. C. at most, the annealing of SiC as heat treatment for activation may be carried out at a high temperature in the range of 1300.degree. C. to 1700.degree. C. since SiC is a thermally stable material even at a considerably high temperature.
FIG. 5 is a graph showing the relationship between the activation rate of implanted impurities and the annealing temperature as reported by Kimoto et al. (Refer to T, Kimoto, O. Takemura, H, Matsunami, T. Nakata, and M. Inoue; J. Electronic Materials, Vol. 27, No. 4, (1998) p. 358.) It will be understood from this graph that approximately 100% activation of impurities cannot be achieved unless the annealing temperature is 1500.degree. C. or higher for aluminum, and 1700.degree. C. or higher for boron.
It is known that such a high-temperature annealing, which is normally performed in an argon (Ar) gas, results in surface roughness due to vaporization of Si atoms on the SiC surface that makes the surface abundant in carbon (C). To avoid this phenomenon, it has been proposed to place the SiC wafer in a SiC container or in a graphite container alone with SiC powder during the high-temperature annealing, so as to produce a Si atmosphere and thus prevent vaporization of Si atoms. With this method, the SiC wafer is able to retain a mirror surface even with high-temperature annealing at about 1700.degree. C.
It was, however, found from careful observation of the SiC surface after high-temperature annealing that a large number of minute defects or irregularities that lead to surface roughness take place even when employing the above method.
For example, the inventors observed the SiC surface that was subjected to high-temperature annealing at 1550.degree. C. with a scanning electron microscope (hereinafter referred to as "SEM") after aluminum ions were implanted in the SiC surface, and found that striped surface roughness occurred in a direction perpendicular to &lt;1, 1, -2, 0&gt; direction. (Refer to Takashi Tsuji, Akira Saito, and Katsunori, Ueno; Extended Abstracts, The 45.sup.th Spring Meeting, 1998, The Japan Society of Applied Physics (30a-YF-1) p. 417). Although a number of minute defects or irregularities that lead to surface roughness are observed where only high-temperature heat treatment is conducted at 1550.degree. C. without implanting ions into the surface, these defects are not as detrimental as those observed after ion implantation. The minute surface roughness appearing after high-temperature annealing is also reported by other researchers (as in M. A. Capano, S. Ryu, M. R. Melloch, J. A. Cooper, Jr., and M. R. Buss: J. Electronic Materials, Vol. 27, No. 4 (1998) p. 370), and it was found that the minute surface roughness becomes more apparent as the annealing temperature becomes higher or the amount of implanted ions increases. It is also known that spot-like defects occur in addition to the surface roughness.
The minute surface roughness is fatal or detrimental to semiconductor devices that utilize their surfaces in their operations. For example, the characteristics of Schottky diodes are greatly influenced by the conditions of the interface between metal and SiC. Also, the interface between an insulating film and SiC is most important to the operation of MOSFET, and it is thus essential to form a high-quality insulating film in the MOSFET.
FIG. 6 is a graph showing an insulating characteristic (denoted by ".smallcircle.") of an oxide film on SiC that was annealed at a high temperature of 1700.degree. C. for 30 min. in Ar gas, after implanting boron ions into the SiC substrate. In the graph, the horizontal axis represents applied voltage, and the vertical axis represents leakage current. The oxide film was formed by a pyrogenic method in which hydrogen and oxygen were supplied. For comparison, FIG. 6 shows an insulating characteristic (denoted by .circle-solid.) of an oxide film of SiC for which no implantation of boron ions nor high-temperature annealing was conducted. The thickness of the oxide film was 35 nm in each case.
The leakage current through the oxide film of SiC on which ion implantation and high-temperature annealing were performed is larger by one or two orders of magnitude than that of the oxide film of SiC on which ion implantation was not performed. Also, the oxide film of SiC subjected to no ion implantation broke down at 45 V or higher, whereas the oxide film of SiC subjected to ion implantation and high-temperature annealing broke down at about 25 V. It is thus understood that the insulating characteristic of the oxide film greatly deteriorates after ion implantation and high-temperature annealing. One of the reasons is that minute surface roughness appears on the SiC surface because of high-temperature annealing.